Recently in Green tech

A solar-powered aircraft has this week been plying the skies around Moffett Field in San Francisco, California, as its inventors rehearse for their next ambitious move: a coast-to-coast sunshine-fuelled flight across the US.

We need a big battery for when the sun doesn't shine (Top Photo Corporation/Rex Features)

A new battery technology may pave the way for cheap, long-lived power storage that can quickly pump electricity into the grid to compensate for fluctuating renewables like wind and solar.

Developed by Yi Cui and colleagues at Stanford University in California, the battery's key advantage is its electrodes, which can run for a thousand charge cycles without degrading. Battery electrodes typically degrade over time as ions in a battery cell repeatedly slam into them and are ripped away again.

By coating the negatively charged cathode in copper hexacyanoferrate and using an anode made of activated carbon and a conductive polymer - compounds that allow electricity-carrying ions to move easily in and out - the team were able to build a prototype battery with electrodes that didn't lose capacity over time.

For more than half a century, researchers have been trying to salvage the substantial amounts of waste heat lost in fossil fuel plants and combustion engines. Heat loss throws away 40 per cent of petrol energy through the car's exhaust, and two-thirds of coal energy from coal-fired power plants.

Their putative ability to mop up that lost energy has made thermoelectric materials a perpetual Cinderella technology. The materials use heat to create "free" electricity: current is generated when the temperature difference between the hot side (say, the exhaust) and the cool side (the ambient air) pushes electrons from one side of the material to the other.

In practice, however, thermoelectric materials reclaim at best only 5 to 7 per cent of the lost energy. Their efficiency - a material's ability to generate electricity for a given amount of thermal energy - is reflected in a figure called its ZT. For 50 years, researchers have struggled to push that number past 1.

Anyone who's dropped a cellphone in the bath knows that water and microelectronics don't usually mix well. But at IBM's Swiss lab in Zurich, marrying the two is becoming almost commonplace: microprocessors with water coursing through microchannels carved deep inside them are already crunching data in SuperMUC, an IBM supercomputer - with the heat that the water carries away used to warm nearby buildings.

And last week, on an unseasonally sunny Zurich rooftop, IBM went public before begoggled journalists with a demo of the technology's newest application: a solar energy-generating microchip array whose waste heat might one day drive desalination systems in arid areas like the Sahara. The firm has long promised this system, and it's still a work in progress, but it has now reached a form that can be demonstrated as part of a collaboration with the Egypt Nanotechnology Center in Cairo.

Windows and smartphone screens may soon come with built in solar power, letting through visible light while gathering energy from other parts of the spectrum.

Researchers led by Yang Yang at the University of California, Los Angeles have built a new type of polymer solar cell which lets through 66 per cent of light at a wavelength of 550 nanometres - green light - and about 60 per cent for the rest of visible spectrum.

The key component of the cell is the top layer - an electrode built by spraying a network of silver nanowires onto a layer of titanium dioxide, then filing in the gaps with nanoparticles of indium tin oxide.

But Yang's production method, known as solution processing, is unique in that it could open the door for making the cells on a large scale using "roll-to-roll" manufacturing techniques - much the same way newspapers are printed on paper.

Polymer solar cells are gaining attention for solar power generation because of their potential to be mass produced more cheaply than traditional silicon photovoltaic cells. However, the efficiency record for turning light into electricity currently stands at 40 per cent for silicon-based cells, while the record for PSCs is just 10.4 per cent, with trade-offs for transparency bringing Yang's efficiency down to 4 per cent.

Hybrid car technology is coming to the Le Mans 24-hour endurance race this weekend as variants of the speed-boosting Kinetic Energy Recovery System (KERS) used in Formula 1 get their first serious workout in another type of competition. Road car makers Audi and Toyota are the first to give the tech a major-league run out at Le Mans, but both are using different KERS technologies.

KERS gives F1 cars a six-second 400 kilojoules (80 brake horsepower) boost once per lap, using kinetic energy harvested during braking to turn a motor-generator - which in turn charges a lithium battery. When the driver pushes a KERS button, the battery discharges back into the motor-generator, turning the wheels faster. Energy storage does not have to be in a battery, however: supercapacitors or a fast-spinning heavy flywheel can do the job, too.

San Francisco has its famous electric streetcars, but Los Angeles looks set to take the concept one step further. Freight trucks may soon buzz up and down electric lines along Interstate 710, which connects Los Angeles to Long Beach, California. This "eHighway" could cut carbon emissions and fuel use by trucks by as much as 30 per cent.

Siemens Technology announced the eHighway project, at the Electric Vehicle Symposium in Los Angeles earlier this month. The company has been testing the cable-truck concept in Germany for some time, but the Los Angeles pilot project would be the first test of the trucks in a real city.

Siemens expects that the cable trucks could begin running within the year.

It's a great city to start in: according to the Los Angeles Times, 40 per cent of the cargo freight that enters the US comes through ports in Long Beach and Los Angeles before being shipped down the highway. Cargo trucks in the US consume nearly 2.5 million barrels of oil per day and improvements to their efficiency lag far behind those to cars.

Once the lines are in place, all eHighway would need is a fleet of hybrid diesel-electric trucks that could switch back to diesel power as soon as they go off the overhead lines. Siemens says that system is nearly seamless: in the test project in Germany, the trucks could switch between diesel and electric power at speeds of up to 90 miles per hour.

The downside, however, is that although electric lines are easy to install on highways and technologically simple, they are very expensive. They may cost as much as five to seven million dollars per mile.

Wind energy proponents have a problem. How can they supply a steady source of power to the grid, even when the wind doesn't blow? The answer may come from an unlikely source - inflatable balloons lashed to the seafloor. Researchers at the University of Nottingham in the UK are currently testing the Energy Bag, a large inflatable energy storage device submerged in water's off Scotland's Orkney Islands.

The concept is relatively simple. Excess electricity from offshore wind farms on windy days is used to run an air compressor, which fills large inflatable bags moored to the seafloor. Then, on calm days, the stored, compressed air can be tapped to drive turbines to produce energy.

Researchers at the Massachusetts Institute of Technology floated a similar idea last year using hollow concrete spheres instead of inflatable bags as a storage vessel. Now the idea of harnessing compressed air on the seafloor is going beyond the drawing board with the current testing off Orkney.

At last, something useful to do with plastic bags. Chemists at the Oak Ridge National Laboratory in Tennessee have found out how to turn the plastic into carbon fibres. So today's plastic rubbish might wind up in tomorrow's carbon-composite racing cars, or in sophisticated filters for chemical processing.

Environmentalists detest plastic bags and some governments have banned or taxed them. Municipal recycling programs generally won't take plastic bags because the chemicals used to make them into films are incompatible with the recycling of plastic bottles. Although stores collect and recycle some plastic bags, most end up being thrown away.

The Oak Ridge team, led by Amit Naskar, has found a way to recycle the polyethylene used in bags and other plastic rubbish into carbon fibres in a wide ranges of sizes and shapes. They mix polyethylene with a compound derived from cornstarch or sugar cane called polyactic acid, heat the mixture, and spin it into bundles of fibres 0.5 to 20 micrometres thick. Each bundle is dipped into an acid-containing chemical bath where it reacts and forms a black fibre that won't melt.

An astronomer famous for designing some of the largest telescope mirrors on the planet now has another trick up his sleeve: he wants to build highly efficient solar collectors that produce electricity on the cheap. An Arizona company he founded, REhnu, claims the new design could drive the price of a solar power farm down to $1 per watt by 2020.

Two decades ago, Roger Angel cast a honeycomb-structure mirror 6.5 metres in diameter at the University of Arizona, using a unique spin-casting technique that rotated a furnace several times a minute, so the molten glass inside formed a parabola close to the mirror's final shape. Five years later, he scaled up to 8.4 metres, and the age of giant Earth-based telescopes was in full swing. Today, four 8.2-metre mirrors designed in this way are used in the Very Large Telescope in Chile. Angel has also proposed making ultra-thin mirrors that could serve as a sunshade to cool the planet.